Truncates the file opened on FILEHANDLE, or named by EXPR, to the
specified length. Raises an exception if truncate isn't implemented
on your system. Returns true if successful,
undef on error.
The behavior is undefined if LENGTH is greater than the length of the file.
The position in the file of FILEHANDLE is left unchanged. You may want to call seek before writing to the file.
Portability issues: truncate in perlport.